Download Performing a Dihybrid Cross

Honors Biology: Performing a Dihybrid Cross
When considering two traits in a cross at the same time, the following
steps are involved:
1. Determine all the combination of genes that can be segregated and
independently assorted into an egg or sperm cell.
2. Write the gene combinations from one parent across the top of the
Punnett square and the gene combinations from the other parent
along the side of the Punnett square.
3. Fill in the squares with ALL FOUR LETTERS.
4. Tally the phenotypic combinations of each type of offspring.
5. Tally the genotypic combinations of each type of offspring. This one
is very time consuming!!!!!
EXAMPLE:
In a particular breed of cat, brown hair (B) is dominant and white hair (b) is
recessive. Short tails (S) are also a dominant trait while long tails (s) are
recessive.
Cross two cats, each of which is heterozygous for both traits, and
determine the phenotype ratio of the offspring. (BbSs x BbSs)
1. Determine the alleles for each cat.
a. Male: BS; Bs; bS; bs
b. Female: BS; Bs; bS; bs
2. Place the alleles across the top and along the side of the Punnett
square; then
3. Fill in the squares with ALL FOUR LETTERS to represent the genes
for both traits (hair color and tail length.)
PROBABILITY—The chance that two events will occur at the same time is
equal to the product of their separate probabilities. This means that it is
much faster to do the previous problem by separating the cross into two
separate crosses. Do the hair color cross on one Punnett square and the
tail length cross on another Punnett square.
Notice that these two Punnett squares result in the same patterns.
Now, to determine the probability that these cats will produce kittens with
brown fur (3/4) and short tails (3/4), multiply the separate probabilities.
3/4 X 3/4 = 9/16
Compare this number to the longer method used on the other side of this
page. Notice that you can get the answer without doing all the tedious
work of a large Punnett square.
Now, imagine hunting for the genotypes. To find the probability that these
cats will produce a kitten that is heterozygous for brown fur and
homozygous for a short tail, you need to search all 16 squares. On the
other hand, the chance of being heterozygous for brown hair is 2/4 and
the chance of being homozygous for short tail is 1/4. This means that the
probability of being heterozygous for brown hair and homozygous for
short tail is:
2/4 X 1/4 = 2/16 (1/8)
Check the Punnett square and you will find two out of sixteen squares fit
this description.
To do a trihybrid cross, you would have to use 64 squares or you can
multiply three fractions. Which would you rather do?